It wasnarrated from Abu Ayyub that the Messenger of Allah (ﷺ)said:
"Whoever fasts Ramadan then follows it with six days of Shawwal,itis as if he fasted for a lifetime." (ID)
English reference (ID): Vol. 1, Book 7, Hadith 1716 (ID)
Arabic reference (ID): Book 7, Hadith 1787 (ID)